This invention relates to a ring-network office recognition method and, more particularly, to a method of recognizing an office in a ring network having first and second network connecting offices for connecting ring networks together and for dropping a signal from a terminal office.
Synchronous optical networks (SONET), which utilize optical communication that is capable of high-capacity transmission, have become widespread owing to an increase in communication traffic. With SONET, user data undergoes multiplexed transmission in accordance with a Synchronous Transport Signal (STS-N) frame (where N represents an integer) format. FIG. 18A is a diagram showing the structure of a 51.84-Mbps STS-1 frame. The frame has 9×90 bytes overall (810 bytes/125 μs), of which 3×9 bytes constitute overhead OH and 87×9 bytes constitute an STS-payload STS-1 SPE (Synchronous Payload Envelope). Nine bytes of the payload constitute path overhead POH, and VT (Virtual Tributary) packets of multiple channels are multiplexed onto the remaining 86×9 bytes. With SONET, frame formats other than the STS-1 frame format mentioned above include STS-3 (155.52 Mbps), STS-12 (622.08 Mbps) and STS-48 (2.488 Gbps). These frame formats can be used in appropriate fashion by optical transmission lines.
Ring Structure
A ring structure in which a transmitting apparatus is connected in the form of a ring from the viewpoint of assuring reliability is known as a network configuration for SONET. The ring structure is such that if a failure occurs in a transmission path, the transmission can be continued via an alternative transmission path, thereby making it possible to improve the reliability of transmission. FIG. 19 is a block diagram illustrating the structure of an ADM (Add/Drop Mux) transmitting apparatus that can be ring-connected. FIG. 20 is a diagram useful in describing the ring structure.
The ADM transmitting apparatus is terminal equipment having a MUX (multiplexing) function and an add/drop function. More specifically, the apparatus has a cross-connect function and an add/drop function for a lower order side (the tributary side). Line interfaces (LINE IF) 1a, 1b receive higher order signals (e.g., OC-48: 2.488-Bbps optical signals) from optical transmission lines 8a1, 8b2 on WEST and EAST sides, respectively, convert these signals to electrical signals and execute processing based upon overhead information. Demultiplexers (DMUX) 2a, 2b demultiplex higher order signals into lower order signals (e.g., STS-1 electrical signals), an STS/VT cross-connect unit 3 performs cross-connect on the STS level, multiplexers (MUX) 4a, 4b multiplex the cross-connected STS-1 signals into higher order signals and line interfaces (LINE IF) 5a, 5b add overhead to these higher order signals, convert the signals to optical signals and send the optical signals to optical transmission lines 8a2, 8b1 on the EAST and WEST sides, respectively. It should be noted that a signal direction in which a signal is input to the EAST side of the transmitting apparatus (node) and output from the WEST side shall be referred to as the EW (EAST→WEST) direction, and that a signal direction in which a signal is input to the WEST side of the transmitting apparatus (node) and output from the EAST side shall be referred to as the WE (WEST→EAST) direction.
The STS/VT cross-connect unit 3 switches, on the STS level, STS-1 signals inserted from tributary interfaces 6a, 6b, . . . via MUX/DMUXs 7a, 7b, . . . and sends these switched signals in the WE or EW direction. The STS/VT cross-connect unit 3 also drops signals, which have been received from the transmission path from the WE or EW direction, on the tributary side, demultiplexes these signals to lower order signals of a prescribed speed via the MUX/DMUXs 7a, 7b, . . . and sends the signals to the tributary side from the tributary interfaces 6a, 6b, . . . . The transmission paths in the WE and EW directions both have working and protection channels assigned to them. For example, in case of OC-48, 1st to 24th channels of the 48 STS-1 channels are working channels and 25th to 48th channels are protection channels. The transmitting apparatus normally transmits signals using the working channel. When a failure occurs, rescue is performed using a protection channel.
Protection at Time of Transmission-path Failure
In accordance with the ring architecture, ADM transmitters 10a to 10d are connected in the form of a ring, as shown in FIG. 20. If a certain transmission path develops a failure or suffers a decline in quality, signals are transmitted in a direction that avoids this transmission path, thereby allowing communication to continue and assuring reliability and quality. Networks in which multiple nodes have been connected into a ring can be classified broadly into two types of schemes, namely a UPSR (Uni-directional Path Switched Ring) scheme and a BLSR (Bi-directional Line Switched Ring) scheme. In comparison with the UPSR scheme, the BLSR scheme is advantageous in that channel capacity can be enlarged because the same channel can be used between different nodes.
The BLSR scheme is such that if failures occur at a plurality of locations and sever a ring transmission path, a signal that cannot reach a destination node may be produced and the signal may be transmitted to another node by loop-back for rescue purposes. In order to prevent such misconnection, squelch is performed. In the squelch operation, a P-AIS (Path Alarm Indication Signal) is transmitted upon inserting the signal, on a per-channel basis, in the signal that cannot reach the target node.
FIG. 21 is a diagram useful in describing rescue from failure. With the UPSR scheme, as shown at (a), the same signal is sent in the EW direction from a node (C) to a node (B) and in the WE direction from the node (C) to a node (D) by, e.g., a channel ch. 1, and a node (A) selects and receives the signal of channel ch. 1 by a path switch PathSW. Accordingly, even if a failure develops between nodes (A) and (B), as shown in (b) of FIG. 21, node (A) is capable of selecting and receiving the signal on channel ch. 1 via node (D) by the path switch PathSW, thereby allowing communication between nodes C and A to continue.
With the BLSR scheme, as shown at (c) in FIG. 21, the node (C) sends a signal node (A) by channel ch. 1 in, e.g., the EW direction and sends a signal to node (D) by channel ch. 1 in the WE direction, and node (D) sends the signal to node (A) by channel ch. 1 in the WE direction. In other words, communication is possible between nodes (C) and (A), between nodes (C) and (D) and between nodes (D) and (A) using the same channel ch. 1. Channel capacity, therefore, can be enlarged as compared with the UPSR scheme.
The BLSR scheme is such that if a failure develops between nodes (A) and (B), as depicted in (d) of FIG. 21, rescue is performed by an ASP (Automatic Protection Switch) protocol using K1, K2 bytes. Specifically, node (B) loops back working channel ch. 1 to protection channel ch. 25 indicated by the dot-and-dash line, and the protection channel ch. 25 is switched over to working channel ch. 1 at node (A), whereby communication between nodes (C) and (A) is allowed to continue. It should be noted that communication between nodes (C) and (D) is performed on channel ch. 1 and that communication between nodes (D) and (A) also is performed on ch. 1 because such communication does not traverse the faulty segment.
FIGS. 22 to 25 are diagrams useful in describing the APS protocol, in which WK represents a working channel and PT a protection channel. Nodes (A) to (H) are connected in a ring configuration by different transmission paths in each of WE and EW direction, and a working channel and protection channel are assigned to each transmission path.
FIG. 22 illustrates a case where communication is performed bi-directionally between nodes (A) and (E). If under these circumstances a failure occurs between nodes (F) and (E) in the transmission path in the EW direction, as shown in FIG. 23, node (E) detects an alarm, becomes a switching node and sends the opposing node (F) switching requests (SF-RING; Signal Failure Ring) 51, 52, which indicate transmission-path failure, in both of short-path and long-path directions, respectively, in accordance with the APS protocol. The switching requests are created using the K1, K2 bytes of overhead (see FIG. 18B). If, upon receiving the switching requests, the nodes (D), (C), (B), (A), (H) and (G) recognize that the destination of request 52 is node (F) and not these nodes themselves, a state of full pass-through is established and the signal is allowed to pass through the protection channel. Upon receiving the request 51 on the short path, node (F) becomes a switching node, sends a reverse request (RR-RING; Reverse Request Ring) over the short path and sends a request 53 (SF-RING), which is identical with the received request 52, over the long path.
In the event of a failure, bridging and switching are executed simultaneously at reception of the request from the long path. Bridging represents a state in which the same traffic is sent by being switched from a working channel to a protection channel, and switching represents a state in which traffic from a protection channel is sent upon being switched to a working channel. Accordingly, owing to occurrence of the failure between nodes (F) and (E), node (E) forms a bridge and sends the signal destined for node (A) to the protection channel PT, as indicated by the dashed line in FIG. 24, and node (F) forms a switch for switching the protection channel PT to the working channel WK from node (F) in the direction toward node (A), as indicated by the dashed line in FIG. 24. The foregoing illustrates rescue of a signal from node (E) to node (A), though a signal from node (A) to node (E) can be rescued in a similar manner. More specifically, as shown in FIG. 25, in this case node (F) forms a bridge for looping back a signal, which was directed from node (A) to node (E) over the working channel, to the protection channel PT, and node (E) performs switching to switch from the protection channel PT to the working channel.
The bytes K1, K2 used in the APS protocol are contained in the section overhead SOH, as shown in FIG. 18B. The K1 byte comprises a switching request of 1st to 4th bits and a remote office ID (the identification number of the node that is the destination of the K1 byte) of 5th to 8th bytes, and the K2 byte comprises a local office ID (the identification number of the node generating the request) of 1st to 4th bits, a 5th bit indicating whether the request is a short-path request (“0”) or a long-path request (“1”), and status of 6th to 8th bits. The switching request of the K1 byte is such that “1011” represents the above-mentioned SF-RING, “0001” represents the RR-RING and “0000” represents no request. If status represented by the K2 byte is “111”, this indicates an AIS (Alarm Indication Signal).
Squelch
Since the same channel can be used by multiple paths in a BLSR network, misconnection of paths occurs if failures develop at multiple locations. In order to prevent such misconnection, the P-AIS (Path Alarm Indication Signal) is inserted, on a per-channel basis, in the signal affected by the misconnection. This operation for inserting the P-AIS is referred to as “squelch”. A squelch table is used to execute squelch. The content of a squelch table specifies the add/drop node of each channel and is set in each node. As shown in FIG. 26A, a node has EAST and WEST sides. The direction in which a signal advances from the EAST to the WEST side through the node is referred to as the EW direction, and direction in which a signal advances from the WEST to the EAST side through the node is referred to as the WE direction. As shown in FIG. 26B, the squelch table describes add/drop nodes in the WE and EW directions with regard to each of the EAST and WEST sides of the node on a per-channel basis. The add node is entered in the source-office name field of the squelch table and the drop node is entered in the destination-office name field. Accordingly, on the presumption that communication is performed bi-directionally between nodes (A) and (E), between nodes (A) and (C) and between nodes (C) and (E), as shown in FIG. 27, squelch tables SQTL-A through SQTL-H of respective ones of the nodes (A) through (H) become as illustrated. It should be noted that these squelch tables have been created using the node IDs of nodes (A) to (H).
Thus, the squelch tables are used to determine whether signals on respective channels can be rescued by loop-back if failures develop at two or more locations in a ring. There is the possibility that a signal judged to be unrescuable based upon the result of the determination made by a squelch table will be output from the wrong node, namely a node different from that intended. Squelch is executed if occurrence of such a misconnection is likely. The node that executes squelch is a switching node, and it does so when failures occur at two or more locations in a ring. Squelch is not executed in the following cases:                (1) when failures have occurred at both ends of the local node (i.e., when the local node is isolated);        (2) when a failure has not occurred on either side of the local node (i.e., when the local node is not a switching node); and        (3) when bridging or switching is not actually being performed.        
Reference will be had to FIG. 28 to describe squelch decision processing at node (E) in a case where failure has occurred between node (E) and (D) and between nodes (F) and (G) simultaneously. If squelch is not executed, a signal on channel ch. 1 from node (A) to node (E) is looped back to the protection channel ch. 25 by a bridging function at node (G), and the protection channel ch. 25 is looped back to the working channel ch. 1 by a switching function at node (D), thereby causing a misconnection in which the signal from node (A) to node (E) is transmitted to node (D). Further, a signal on channel ch. 1 from node (E) to node (C) is looped back to the protection channel ch. 25 by a bridging function at node (E), and the signal is looped back to the working channel ch. 1 by a switching function at node (F), thereby causing a misconnection in which the signal from node (E) to node (C) is transmitted to a lower order group via node (E).
Accordingly, if multiple failures have occurred, (1) the locations of the failures are identified, (2) nodes at which signals will not arrive from the faulty locations are found from the ring topology, (3) reference is had to the squelch tables to determine whether the nodes that have been entered in these tables are nodes at which signals will not arrive, and (4) if a node is one at which a signal will not arrive, then squelch is executed.
Ring topology is the topology obtained by arraying the names of nodes that construct the ring clockwise in order starting from the node of interest. FIG. 28 illustrates ring topology RTG of node (E). It is ascertained from the faulty locations and ring topology RTG of FIG. 28 that nodes at which signals will not arrive are the nodes of node IDs 9, 6, 4, 1, 14, 3. It is determined whether source and destination nodes that have been entered in a squelch table SQTL-E of node (E) match nodes at which signals will not arrive. Since it is found that node (C) of node ID 14 and node (A) of node ID 4 are nodes at which signals will not arrive, squelch is executed. In other words, squelch is executed at the switching nodes (D), (E), (F) and (G) by inserting P-AIS in each of the channel signals after bridging and after switching.
FIGS. 29A and 29B are diagrams useful in describing single failure and multiple failures. FIG. 29A is for a case where a single failure has occurred and FIG. 29B for a case where multiple failures have occurred.
If a single transmission-path failure SF occurs between nodes (F) and (E), as shown in FIG. 29A, and node (E) detects the failure SF (Single Failure), the node (E) operates in a manner similar to that described above in connection with FIG. 23. That is, node (E) (1) sends a switching request 61 (SF-R/F/E/Long) in the long-path direction to the node (F) of the opposing office and (2) sends a switching request 62 (SF-R/F/E/Srt/RDI) in the short-path direction to the node (F). In the event of the single failure, node (F) receives the short-path and long-path switching requests 61, 62, which are destined for its own office, sent from the node (E) of the opposing office, and therefore determines the occurrence of a single failure in one direction between the nodes (F) and (E).
If a failure occurs between nodes (B) and (C) and between nodes (F) and (E), as shown in FIG. 29B, node (C) detects a transmission-path failure SF1 and node (E) detects a transmission-path failure SF2, then nodes (C), (E) generate request signals. Specifically, in response to detection of failure SF1, node (C) sends a long-path switching request 63 (SF-R/B/C/Long) to the node (B) of the opposing office and sends a short-path switching request 64 (SF-R/B/C/Srt/RDI) to node (B). Upon receiving the short-path request 64, node (B) sends a long-path switching request 65 (SF-R/C/B/Long) to node (C).
Further, upon sensing the transmission-path failure SF2, node (E) sends a long-path switching request 66 (SF-R/F/E/Long) to node (F) of the opposing office and sends a short-path switching request 67 (SF-R/F/E/Srt/RDI) to node (F) in a manner similar to that of the single-failure case of FIG. 29A. In response to receipt of the short-path switching request 67, node (F) sends a long-path switching request 68 (SF-R/E/F/Long) to node (E). Node (F), from the fact that the long-path switching request 65 (SF-R/C/B/Long) is not destined for its own office but is directed from node (B) to node (C) of another office despite the fact that the short-path switching request 67 (SF-R/F/E/Srt/RDI) is directed to its own office from node (E), judges that failures (multiple failures) have occurred between nodes (F) and (E) and between nodes (B) and (C).
Construction of Ring Topology
FIGS. 30A to 30C are diagrams useful in describing the construction of a ring topology.
In a system in which four nodes (A) to (D) are connected by a ring transmission path RL, an identification number is assigned to each node, as shown in FIG. 30A. For example, 15, 3, 7 and 8 are assigned as the IDs of nodes (A), (B), (C) and (D), respectively. Next, as shown in FIG. 30B, (1) node (A), which specifies the construction of the ring topology (ring map), sends a ring topology frame RTGF, in which the inserted-node number is 1 and ID 15 of its own node is assigned to the first field. The ring topology frame RTGF is sent in the clockwise direction, by way of example. (2) Next, node (B) sends a ring topology frame RTGF, in which the inserted-node number is 2 and the ID of its own node is inserted following the ID of node (A). (3) Similarly, node (C) sends a ring topology frame RTGF, in which the inserted-node number is 3 and the ID of its own node is inserted following the ID of node (B), and (4) node (D) sends a ring topology frame RTGF, in which the inserted-node number is 4 and the ID of its own node is inserted following the ID of node (C).
(5) Since the first inserted-node ID is its own node ID, node (A) recognizes that the frame has come full circle and, as shown in FIG. 30C, transmits the ring topology frame RTGF upon inserting an END flag at the end thereof, whereby each node is notified of the completed ring topology frame. Each node that has received this ring topology frame constructs a ring topology with its own node at the head. For example, the topology is “15, 3, 7, 8” at node (A), “3, 7, 8, 15” at node (B), “7, 8, 15, 3” at node (C) and “8, 15, 3, 7” at node (D). Such a ring topology makes it easy to send a local node ID and a target node ID using the K1, K2 bytes in accordance with the APS protocol.
FIGS. 31 to 35 are diagrams useful in describing the formation of squelch tables. The nodes (A) to (D) have respective ones of squelch tables storing node IDs. For the sake of simplicity, however, characters the same as those of the nodes (A) to (D) will be used as the node IDs. Further, each squelch table has a structure for specifying an add/drop node with regard to the EW and WE directions, as shown in FIG. 26A. However, to simplify the description, add/drop nodes only in the EW diction are shown. If signals are sent and received in the EW direction between the nodes (C) and (D) via the nodes (B), (A), add node (C) inserts its own node ID “C” into a squelch table SQTL-A and then transmits the table to the side of node (B), as indicated at (1) in FIG. 31, thereby reporting the fact that node (C) is an add node. Further, drop node (D) also inserts its own node ID “D” into a squelch table SQTL-D and then transmits the table to the side of node (A), thereby reporting the fact that node (D) is a drop node. It should be noted the asterisk and star marks signify that the other parties are unknown.
Next, as indicated at (2) in FIG. 32, node (A) notifies node (B) that the drop node is node (D), and node (B) notifies node (A) that the add node is node (C). Next, as indicated at (3) in FIG. 32, node (B) notifies node (C) that the drop node is node (C), and node (A) notifies node (D) that the add node is node (C). Thus, the local node ID (=“C”) and the drop node ID (=“D”) of the opposing office are set in the WEST fields of the squelch table SQTL-C of the add node (C), and the local node ID (=“D”) and the drop node ID (=“C”) of the opposing office are set in the EAST fields of the squelch table SQTL-D of the add node (D).
Next, on the basis of the completed squelch tables, node (C) notifies node (B) that the unknown party * is node (D), and node (D) notifies node (A) that the unknown party P is node (C), as indicated at (4) in FIG. 34. Finally, node (B) notifies node (A) that the unknown party * is node (D), and node (A) notifies node (B) that the unknown party P is node (C), as indicated at (5) in FIG. 35. As a result, squelch tables SQTL-A, SQTL-B are completed also at nodes (A) and (B).
In a case where one optical-fiber transmission line has working channels ch. 1 to ch. 24 and protection channels ch. 25 to ch. 48, channels that are rescued by loop-back at the occurrence of failure are only the working channels ch. 1 to ch. 24. Accordingly, it will suffice to create squelch tables solely for the working channels ch. 1 to ch. 24.
2-Fiber BLSR Scheme and 4-Fiber BLSR Scheme
As shown in FIG. 36A, a 2-fiber BLSR scheme uses one transmission line (fiber) in each of the WE and EW directions. A working channel and a protection (or extra) channel are assigned to each transmission line. If a failure occurs in the working channel of one transmission line, loop-back is performed so that the signal is transmitted via the protection channel of the other transmission line.
As shown in FIG. 36B, a 4-fiber BLSR scheme provides two transmission lines, namely a working transmission line and a protection transmission line, in the WE direction, and two transmission lines, namely a working transmission line and a protection transmission line, in the EW direction, and transmission is performed using these four transmission lines.
The 4-fiber BLSR scheme differs from the 2-fiber BLSR scheme as follows:
(1) The working and protection channels are accommodated in different fibers.
With the 2-fiber scheme, protection is performed using one-half the working channel. As a consequence, traffic for maintaining the band must be accommodated in less than one-half the total. With the 4-fiber scheme, all traffic can be looped back using the protection fiber. However, under ordinary conditions only extra traffic can be passed through the protection fiber.
(2) With the 4-fiber BLSR scheme, a span switch is used. For a failure that cannot be rescued by the span switch, loop-back is performed using a ring switch.
The 4-fiber BLSR scheme is such that if a failure occurs only in a working fiber, as shown in (a) of FIG. 37, span changeover is performed using a span bridge switch SPB, a span switch SPS and the protection fiber. However, a span changeover is not carried out even a failure occurs in the protection fiber. Further, with the 4-fiber BLSR, loop-back is performed by ring switches RSW1, RSW2 if failures occur simultaneously in two fibers of the same direction, as illustrated at (b) in FIG. 37.
Service Selector SS
A BLSR network can be expanded up to a maximum of 16 nodes; more than 16 nodes cannot be accommodated. This limitation on network expansion can be avoided by connecting a certain BLSR network A and another BLSR network B via a ring interconnection. Ordinarily, as shown in FIG. 38, a ring interconnection is implemented by interconnecting a tributary (lower order channel) of a prescribed node E in the BLSR network A and a tributary of a prescribed node C′ in the other BLSR network B. With this approach, however, any failure that might occur between the connected nodes E and C′ cannot be rescued and would result in loss of communication between the rings.
Accordingly, line survivability in the event of node failure is enhanced by interconnecting two tributary connection offices in each of the BLSR networks A and B, as illustrated in FIG. 39. In accordance with this connection, service selectors (SS1, SS2) of primary nodes in respective ones of the networks perform switching (path switching) of a signal dropped from another network and a signal that enters from a secondary node within the same network. This is for the purpose of performing rescue in the event of a failure. For example, a signal of a prescribed channel that has been sent from a node (A)′ of BLSR network B enters the service selector SS2 of the BLSR network A via a drop side and a continue side. The service selector SS2 normally selects the channel signal that enters from the continue side and sends this signal to the node (A). If a failure in the transmission path occurs at point F under these conditions, the service selector SS2 subsequently selects the channel signal that enters from the drop side and sends this signal to node (A) to continue communication.
Thus, in accordance with the connection scheme shown in FIG. 39, rescue in the event of a failure between the BLSR networks A and B can be implemented by the path switching function and a path setting function, which is referred to as DCW (Drop & Continue on Working Bandwidth), of the service selector switches SS1, SS2. However, in order to realize the ring interconnection with the connection method of FIG. 39, a working channel must be used in the connection between the primary office and the secondary office. As a consequence, the connection method of FIG. 39 detracts from line use efficiency, which is one advantage of the BLSR network, meaning that the characteristics of the BLSR network cannot be exploited effectively.
With regard to this problem, the ITU-T recommends that the connection between the offices of the ring interconnection within the same network be implemented not by a working channel but by a protection channel, as shown in FIG. 40.
In the recommended connection arrangement, a primary office (node A-1) drops a signal (an added signal from a lower order group) from a terminal office (node A-n) within the same BLSR network A and uses the protection channel to deliver the signal from the terminal office to the secondary office (node A-2). Further, the primary office (node A-1) forms a path switch (service selector) SS between a signal (an added signal from the lower order group) from a terminal office (not shown) within a different network B and a signal received from the secondary office (node A-2) using the protection channel, and forms a cross-connect that delivers a signal of excellent quality to the terminal office (node A-n) within the same network. Further, the secondary office (node A-2) forms a cross-connect that drops a signal received from the primary office (node A-1) via the protection channel and delivers an added signal from the lower order group to the primary office (node A-1) via the protection channel.
Because of the characteristics of a BLSR network, a line that uses a protection channel is referred to as a PCA (Protection Channel Access=Extra Traffic) line and has a low priority. If any node performs line rescue when a failure occurs in the network, the PCA line becomes the rescue line and communication via this PCA line can no longer be carried out. In other words, if the connection method of FIG. 40 is implemented as is without changing the conventional BLSR function, the specially provided ring interconnection becomes meaningless. That is, even through the line in question can be rescued, there is a possibility that that the line will not be rescued.
The ITU-T recommendation merely recommends a ring interconnection using a protection channel but does not specifically state how this should be implemented; the method of implementation is left to the communication service provider.